1. Field of the Invention
This invention relates to land mobile radio systems and more specifically to minimizing interference from adjacent spectral frequencies in land mobile radio systems.
2. Description of Related Art
Conventional land mobile radio (LMR) channels employ narrow-band frequency division multiplexed (FDMA) systems with different radio units assigned to different frequency bands. These bands are typically 25 KHz wide. There is an immediate need for an increase in capacity of LMR systems in the U.S. for such applications as public safety trunking. The trend is to increase capacity by splitting each existing 25 KHz channel used in LMR systems into two 12.5 KHz channels. However, this results in increased adjacent channel interference (ACI), unless there is a considerable reduction in data rate, which is undesirable. ACI is interference introduced at a receiver from a transmitter broadcasting at a frequency corresponding to an adjacent channel and is sometimes called adjacent channel `splatter`.
In a typical LMR system, communication between mobile units takes place through a base unit (base station). Each base station serves a certain geographic area. Communication between mobile units and base units takes place on a pair of frequencies that are separated, usually widely, to prevent interference. One frequency is needed for base to mobile communication and the other frequency in the pair is used for mobile to base communication. In some situations mobile units can communicate with each other directly without going through the base unit. This is called "Talk-Around". A pair of frequencies are also used in Talk Around communications, one for each direction.
A problem occurs if two mobile units employ spectrally adjacent frequencies to communicate with their base units. Usually, mobile units within the same area will not be assigned spectrally adjacent frequencies but mobiles in contiguous geographic areas can use spectrally adjacent frequencies. The same situation exists with respect to frequency assignments to base units.
A measure of how well a system resists ACI is an ACI protection ratio (ACIPR). For analog FM the ACIPR is specified to be in the range of 65-70 dB. Some digital modulation schemes offer adequate spectral efficiency but lower ACIPR (in the range of 45-50 dB). The ACIPR values may be augmented by several techniques. However, the problem gets increasingly difficult as the need for capacity and higher spectral efficiency arises.
Some of the commonly adopted techniques to improve ACIPR for digital modulation are antenna diversity in which more than one antenna receives a signal and the receiver chooses the signal from the antenna having a better signal strength. Antenna diversity is useful in providing a margin of 3-5 dB in ACIPR. Antenna diversity is further described in Characterizing the Effects of Nonlinear Amplifiers on Linear Modulation for Digital Portable Radio Communications, by S. Ariyavisitakul and T. P. Liu, IEEE Transactions on Vehicular Technology, Vol. 39, No. 4, pp. 383-389, November, 1990.
Another technique to improve ACIPR is interference rejection and cancellation where an estimator is employed in estimating what a signal should be, and subtracting the estimated signal from the actual signal to synthesize an interference signal which is then subtracted from the further received signals. A similar technique is interference rejection using filtering described in Rejection Method of Adjacent Channel Interference for Digital Land Mobile Communications, by S. Sampei and M. Yokohama, The Transactions of the IECE of Japan, Vol. E 69, No. 5, pp. 578-580, May 1986. Interference cancellation is described in Method of Rejecting Adjacent Channel Interference Using an Adaptive Equalizer, by N. Kinoshita and S. Sampei, Transactions of IEICE (section B), J71-B, 10, pp. 1119-1126, October, 1988. Interference rejection and cancellation involves complex receiver circuitry and is highly dependent upon the channel conditions and interference power. These techniques can provide up to 6-10 dB of gain if properly implemented.
Transmitter power control is described by Y. Nagata and Y. Akaiwa in Analysis for Spectrum Efficiency in Single Cell Trunked and Cellular Mobile Radio, IEEE Transactions on Vehicular Technology, Vol. VT-35, No. 3, pp. 100-113, August, 1987. Transmitter power control offers a larger gain (10-15 dB) in ACIPR by controlling the transmit power of mobile stations. In such transmitter power control, the mobile units which are closer to the base station transmit at a lower power in order not to "splash" other mobile units. The base station power is not varied. This scheme is complex and the complexity increases with capacity.
Optimization of transmitted in-band to adjacent band power has been achieved by choosing a transmitted code which minimizes ACIPR as described in Trellis Coding Technique to Improve Adjacent Channel Interference Protection Ratio in Land Mobile Radio Systems Ser. No. 07/898,670 by S. Chennakeshu, A. Hassan and J. Anderson and Improved Trellis Coding Technique to Improve ACIPR in Land Mobile Radio Systems Under Peak Power Constraints (Ser. No. 07/975,201) by Sandeep Chennakeshu, R. Ramesh, Amer A. Hassan and John B. Anderson, both filed Jun. 15, 1992. However these techniques did not address transmit filter optimization for maximization of ACIPR.
Maximization of in-band power alone using optimized transmit and receive filters was described in Optimum FIR Transmitter and Receiver Filters for Data Transmission over Band-Limited Channels by P. R. Chevillat and G. Ungerboeck, IEEE Trans. on Commun. Vol. 30, No. 8, pp. 1909-1915, August, 1982. The problem with this technique is that it does not account for adjacent channel power relative to in-band power and hence does not provide adequate ACIPR optimization.
Currently, there is a need for a spectrally efficient modulation scheme that exhibits high ACIPR and offers a transmit range comparable to existing analog FM systems.